1. FIELD OF THE INVENTION
This invention relates to a highly integrated LADAR/FLIR sensor for target acquisition for use on a missile at low altitudes or on aircraft.
2. BRIEF DESCRIPTION OF THE PRIOR ART
The laser radar (LADAR) and forward looking infrared (FLIR) sensors are complementary sensors in target acquisition systems. The FLIR images the infrared light emitted from a scene in an object plane and senses the image with an array of a hundred or more detectors. The image is then processed by a computer programmed with target detection algorithms to locate areas in the field of view that may contain targets. Targets, such as tanks or trucks, are generally warmer than the background due to the heat generated by the engine and to solar heating and therefore can be seen in a FLIR image. The FLIR image (sometimes called a "thermal image") of the target can vary greatly due to differences in solar heating from day to night and due to differences in engine loading.
A scanning FLIR sensor can quickly image large areas due to the large number of detectors in the array. Since targets have contrast with respect to the background, a FLIR can detect the presence of targets in the scene. However, the variability of the thermal image of a target makes it difficult to classify a target with respect to type (e.g. tank, truck or armored personnel carrier) with a target classification algorithm.
A laser radar (LADAR), on the other hand, creates a three dimensional image of a scene. A short pulse of laser light is transmitted to the scene and is reflected back to the LADAR which detects the return light. The duration of the interval in time between transmission and reception of the light is measured and related to range or distance to the scene. The LADAR is scanned over the scene and the range to each location in the scene is measured. In this manner, a three dimensional image of the scene is formed. A LADAR generally transmits a single beam and has a single detector to receive the return. Hence, a single LADAR cannot quickly image large areas as does a multiple detector FLIR. Therefore, a LADAR is not very useful in rapidly searching large scenes for potential targets. However, the three-dimensional target image created by the LADAR is stable in that it does not vary with engine loading or time of day. Hence, the three-dimensional image is useful for target classification in comparison with the thermal image.
The FLIR and the LADAR are complementary sensors in a target acquisition system. The FLIR creates a thermal image of a large scene, also known as a field of regard, quickly. A processor programmed with target detection algorithms processes the thermal image and locates potential targets. The LADAR is successively pointed to each of these potential targets, which is a small part of the field of regard, and creates a three-dimensional image of the potential target. A processor programmed with target classification algorithms processes the three-dimensional images and classifies the potential targets according to type. The classification algorithms can also declare the potential target not to be a target at all. This architecture makes use of the rapid imaging and target detection capabilities of the FLIR and the classification capabilities of the LADAR.
If the FLIR/LADAR sensor is to be small, light and rugged, the LADAR and FLIR hardware must be carefully integrated. A straightforward approach is depicted in FIG. 1. A FLIR and LADAR are mounted on a platform so that their optical axes are parallel. The optical axes of the two sensors are made to coincide with a beam combiner, such as a dichroic beam splitter. Here, the dichroic beam splitter transmits the passive infrared light forming the FLIR image and reflects the light from the LADAR. A head mirror is mounted in front of both the LADAR and FLIR and is capable of pointing either sensor in a selected direction. The head mirror is used to point the FLIR to the location in the object plane which is to be searched for targets. The thermal image of the scene is created and the processor detects the location of potential targets in the scene. The head mirror then points the LADAR toward the potential targets and a three-dimensional image is created. A processor programmed with target classification algorithms then processes the three-dimensional images to classify the potential targets.